1. Field of the Invention
The present invention relates generally to servo systems for controlling the position of one or more read/write heads relative to the different concentric tracks on a disk drive employing a sector servo system.
2. Prior Art
In disk drives employing sector servo information, in which sectors of servo information are interspersed with blocks of data circumferentially in concentric tracks around the surfaces of the disk or disks, a servo format pattern is usually employed which contains information such as a track number identifying field, a synchronizing field which synchronizes the data reading devices with the data rate on the disk drive, and a servo burst field for providing information to control the positioning of the heads which read and write information from and to the disk surfaces.
One operation phase of a disk drive servo system is known as track seeking, in which the read/write head is driven across the disk from one track to another. By controlling the head motion velocity profile between tracks, such track seeking can be performed quite rapidly compared to other systems such as those with a stepper motor actuator, where the head has to step through (i.e., start and stop) at essentially a constant velocity. This track seeking operation in a sector servo system can be conducted most efficiently when the recorded servo block information includes such lateral positional information as track ID code, and the blocks are placed in a phase coherent fashion.
The operation phase of a disk drive servo system in which the read/write head is maintained over the centerline of the desired track is known as track following. During operation, the head position can be shifted from its desired position by a number of factors, such as the application of external force to the head. The servo system senses this drift in position by reading the servo bursts and comparing their amplitudes, and then corrects for the drift by moving the head back to the desired track centerline.
One prior art servo burst field pattern for use in disk drives, as shown in U.S. Pat. No. 4,669,004, contains four bursts of servo information labeled A, B, C, and D. All like bursts are separated from each other by four/thirds of one data track pitch. Each burst type is offset radially from the next adjacent other burst types by either one third or two thirds of the pitch of each data track. Since the actual width of a head is approximately two thirds of a track pitch, approximately one third track pitch nominally separates each data track from adjacent tracks. The servo burst pattern repeats itself every fourth track.
For each data track in this prior art servo pattern, two of the four bursts are equally offset from a track centerline by one third track pitch. For that track, one of the other bursts is entirely within the head width at that particular track, and the other is entirely outside the head width. This arrangement provides a quadrature pattern, so that the relative amplitudes of bursts read by the head at a known position will provide direction feedback information during track seeking operations. The position of the head is determined using the four servo bursts and conventional quadrature mathematical analysis techniques.
One disadvantage of this prior art four burst servo pattern is that it requires additional area on the disk to store the servo pattern in comparison to that required for a pattern having two or three servo bursts per sector. This additional area required for the servo pattern subtracts from the disk surface available for storing user data, thereby reducing the overall data capacity of the disk file.
Another prior art patent, U.S. Pat. No. 4,297,787 shows a sector servo pattern having two servo bursts in the servo field and utilizing a SYNC field following the servo bursts, the SYNC field being encoded to indicate whether the preceding servo burst information was valid or invalid.
Servo information is generally written in a form of a number of separate bursts of sine wave signals, each of which is physically written a half track away from adjacent bursts. For example, in the system of the present invention, three servo bursts, A, B and C are written, with the C burst written exactly on a track centerline, the A burst written a half track inside of the centerline, and the B burst written a half track outside the centerline. When these bursts are read with the head exactly on the track centerline, the system reads a normalized amplitude of 1.0 for the C burst and 0.5 for each of the A and B bursts. By comparing the amplitude differences of the A, B and C burst signals, the servo system can determine exactly where the head is with respect to the track centerline, and the head location may be adjusted by control of the head positioning actuator.
The servo system which this invention implements has servo bursts between each sector on the disk. Therefore, servo information must be written in such a manner that is readily distinguishable from the user data. The function of detecting such specially written information is called discrimination. In such a system, because the servo information also identifies the location of the user data area immediately following it, the discriminator must accurately detect the servo field in order for the system to be able to read/write in the data portion of the sector. Therefore, any false detection or mis-detection can not only affect the system throughput rate, but could also result in a loss of data.
Traditionally, such discrimination function is implemented in analog circuitry whose performance can be affected by environmental factors (temperature, humidity, power supply voltage, etc.) and by component variations. Additionally the discriminating criteria of such analog circuitry tends to be non-exact, which usually requires high precision components and costly factory adjustment.
A desirable alternative is to perform such discriminating function in digital logic circuitry which is more stable against environmental variations and in which detection criteria can be controlled in a more exact fashion.